It could be any number of things, though the primary reasons would seem to be #1 alcohol, #2 religious guilt, #3 manophobia. Seems some guys just can't perform when another man is around. And some people have so much psychological baggage from religio they can't perform. If a man you are with can't perform, ask him to give you oral for an orgasm or two or three and hand him a dildo to service you with. Hel, using toys always gets me hard again.
J